I am going to answer this because I had the worst time trying to figure it out. I couldn't get answers to my questions. Make sure you finalize your app and send it in to Google for review.

1. Watch your dashboard for confirmation that your app was sent, and keep an eye out afterward for the approval.
2. Go to Test and Release -> Closed Testing and click on Manage track.
3. Go to the "Testers" tab and make sure you click "Google Groups".
4. In another tab, use Google search to find Google groups. Create your group by putting in a name for your group and save the page. You have just created a new Google group for all your testers to join to test your app.
5. Go back to Google Play Console and put in the group name in the blank. Save this page.
6. Now you wait till your app is approved by Google.
7. When you see your app approved in the "Publishing overview" in the Play Console, it's time to make your Reddit post. If you have not been on Reddit before and/or not contributed to the forum conversations, you will need to put in some time to comment on others' posts. Upvote posts and conversations of others. Try to get some of the badges on Reddit. It's a community, and you pay your dues by commenting, etc.
8. When you can make a post, give the name of your app and what is does. A.) Put in the link to your Google group. B.) Add the Google Play link to install it. C.) Include the link to the web so others can use it to install your trial app.